Marine seismic survey for exploring undersea natural resources such as petroleum, natural gas, gas hydrate, or the like, or bedrock investigation for marine construction such as undersea pipeline and cable burial, undersea tunnel, undersea storage equipment, bridge, or the like, has been performed.
For the purpose of resource exploration, a need exists for a large-capacity air gun operated at a low frequency and a long streamer to several km in length so as to understand a geological structure to several km in depth.
However, since the marine seismic survey for the purpose (generally, referred to as “engineering purpose”) of marine construction such as undersea pipeline and cable burial, undersea tunnel, undersea storage equipment, bridge, or the like, is performed to understand a subsurface geological structure of a shallow area, a seismic source generator such as a small-sized air gun of a high frequency, a sparker, a boomer, or the like, and a streamer (a single channel streamer or a small-sized multi-channel streamer) are used.
Further, it is difficult to perform the exploration for the engineering purpose using a large prober when water is shallow in a survey area. Therefore, the exploration for the engineering purpose is mainly performed by a small ship, a fishing boat, or the like, and needs to be economically performed within a budget.
A marine seismic survey data acquisition system may be configured to include a GPS, a seismic source generator, a streamer, a recording system, or the like.
The GPS for the marine seismic survey performs positioning of ships or all the equipment while performing data acquisition. Further, a differential GPS (DGPS) has been used to perform accurate positioning.
The seismic source generator artificially generates seismic signals. A representative example of the seismic source generator used in marine survey for the engineering purpose may include an air gun, a sparker, a boomer, or the like.
As the related art relating thereto, there is Japanese Patent Laid-Open Publication No. 2007-236005 (Sep. 13, 2007).
Generally, the broader the frequency band of the seismic source generator, the higher the resolution becomes. However, the seismic source generator is not transmitted to a deep softground.
The seismic source receiver is an apparatus that receives the seismic signals artificially generated by the seismic source generator.
The recording system is an apparatus that converts and stores the received signals from analog to digital.
The air gun used for the marine petroleum exploration, which is an apparatus suddenly discharging high-pressure air into the water to generate a sound wave, has been the most prevalently used for the marine survey since it has a frequency of several tens of Hz.
However, the air gun requires numerous accessories such as a compressor capable of continuously generating and supplying the high-pressure air, a large-capacity generator, or the like, a working space, and high cost, which is not appropriate for the engineering exploration using a small ship.
The above-mentioned sparker, which generates a spark by high-voltage current applied between two electrodes mounted into the water, has a type that generates vapor bubbles at the time of generating spark to generate a primary pulse.
When the bubbles are cooled until they are shrunk, secondary bubble pulses are generated. The secondary bubble pulses have a frequency of 20 to 200 Hz, which has been mainly used to investigate a seafloor or a strata of which the depth is shallow.
The boomer instantly discharges voltage to generate energy, which is very suitable for the engineering exploration.
The boomer is a type that generates an acoustic pulse caused by the deformations of the piezoelectric transducer when electric energy is discharged to a coil and eddy current is generated to the coil from a transducer (referred to as ‘plate’).
Since the boomer has a frequency of 400 to 1300 Hz, the boomer is mainly used to investigate the seafloor or the strata of which the depth is shallow, similar to the sparker, but since the boomer includes a larger amount of high frequency components than the sparker, the boomer may obtain a more precise seismic cross section.
FIG. 1 is a schematic diagram for explaining marine seismic survey using a boomer.
The boomer having one transducer and four transducers generating signals has been used. The boomer having one transducer is generally referred to as a bubble pulser and the boomer having four transducers are generally referred to as ‘quad pulser’.
The bubble pulser has one transducer, which is relatively easy in view of the movement and operation. However, the bubble pulser has smaller energy than a water level at a place where the water is deep, thereby degrading the quality of seismic reflection data.
Since the quad pulser has four transducers, the seismic reflection is clearly shown even at the place where the water is deep, but is heavy (about 200 kg to about 20 kg per one transducer), such that it is difficult to move and operation the quad pulse by manpower.
Although there is a need to provide the appropriate number of transducers according to situations in consideration of survey purpose, water level, working space, working conditions, costs, or the like, while solving the difficulty in the above-mentioned movement and operation, the boomer according to the related art may not satisfy them.
When forcibly removing some of the four transducers included in the boomer according to the related art, the center of gravity of the boomer may be biased to one direction, or the like, such that it may be difficult to separate and use some transducers in the boomer according to the related art.